Digital light sensing based upon solid state technology is well known, the two most common types of light sensors currently being charge coupled devices (CCD's) and complementary metal oxide semiconductor (CMOS) light sensors. Digital light sensors are incorporated within a wide variety of devices throughout the consumer, industrial and defense sectors among others.
A light sensor is a device comprising one or more radiation sensitive elements having an electrical property that changes when radiation is incident upon them, together with circuitry for converting the changed electrical property into a signal. As an example, a light sensor may comprise a photodetector that generates a charge when radiation is incident upon it. The photodetector may be designed to be sensitive to electromagnetic radiation in the range of (human) visible wavelengths, or other neighboring wavelength ranges, such as infra red or ultra violet for example. Circuitry is provided that collects and carries the charge from the radiation sensitive element for conversion to a value representing the intensity of incident radiation.
Typically, more than one radiation sensitive element will be provided in an array. The term pixel is generally used as shorthand for picture element. In the context of digital light sensing, a pixel refers to that portion of the light sensor that contributes one value representative of the radiation intensity at that point on the array. These pixel values may be combined to represent light sensed by the sensor.
Pixels are usually formed on and/or within a semiconductor substrate. In fact, the radiation sensitive element comprises only a part of the pixel, and only part of the pixel's surface area (the proportion of the pixel area that the radiation sensitive element takes up is known as the fill factor). Other parts of the pixel are taken up by metallization such as transistor gates and so on. Other image sensor components, such as readout electronics, analog to digital conversion circuitry and so on may be provided at least partially as part of each pixel, depending on the pixel architecture.
When sensing levels of ambient light it is desirable for light from a large area to contribute to the overall level of ambient light sensed. That is, an ideal ambient light sensor can be said to take in light from across an entire hemisphere of illumination surrounding an input window of a sensor package. Ambient light sensors therefore require a large (or wide) field of regard. The wider the field of regard, the more accurate the measured level of ambient light may be.
An ambient light sensor generally comprises a single radiation sensitive element such as a photodiode; however ambient light sensors may comprise a plurality of radiation sensitive elements. Each radiation sensitive element is termed a pixel. The pixels may form part of a charge coupled device (CCD) or complimentary metal-oxide semiconductor (CMOS) light sensor. Other components such as lenses and mounts may be added to produce an ambient light sensor package for installation in an electronic device.
Generally, ambient light sensors may comprise a low number, e.g. one, four, nine or sixteen, of radiation sensitive elements or pixels arranged in a square matrix on a substrate such as a silicon substrate. However, ambient light sensors with more pixels may be envisaged. Each pixel is arranged to abut adjacent pixels to leave no space in between pixels. The pixels are therefore arranged in a pixel array. A pixel array is a type of sensor array specific to light sensors.
Ambient light sensors are configured to give an indication of the ambient light surrounding a device. Information concerning ambient light may be used e.g. to control the brightness of a display on a device. Ambient light sensors are not generally required to produce an image and are required to be small so as not to occupy valuable space in a device. They are therefore required to have small pixel arrays when compared to the large pixel arrays, e.g. 2 megapixel, of camera module microchips.
The term “field of regard” is used herein to encompass the maximum angle from which light incident on a sensor package may be detected by pixels in a pixel array. In the case of an ambient light sensor package, the light incident on the pixel array is used to measure the level of ambient light. A field of regard may be expressed in terms of a half-angle of a viewing cone. That is an angle may be defined from a normal, which is perpendicular to a pixel or pixel array; the angle defines the maximum angle from which light may be incident on the pixel or pixel array.
It is noted that the shape of a viewing cone that defines a field of regard is dependent on the shape of a pixel array to which it relates. That is, if the pixel array is square, then the viewing cone will form an inverted square based pyramid. If the pixel array is circular, then the viewing cone will form an inverted circular based pyramid.
The field of regard is therefore defined by the interrelationship between a pixel array and a lens forming part of a sensor package. Taking the ideal ambient light sensor package example referred to above in which light is sensed across an entire hemisphere of illumination, the half-angle would be ninety degrees. In other ambient light sensor packages, the half-angle may be less than ninety degrees.
The term “pixel” as used herein encompasses an individual radiation sensitive element or sensor element arranged on a substrate. The term “sensor array” as used herein encompasses an array of sensors arranged on a substrate. A pixel array is a type of sensor array. The term “sensor array microchip” as used herein encompasses a sensor or a sensor array and a substrate on which the sensors are arranged. The term “sensor package” as used herein encompasses a collection of elements that together form at least part of a complete sensor module or unit suitable for installation in a device. For example, an ambient light sensor package may comprise a sensor array microchip comprising pixels arranged in a pixel array on a substrate, a lens, a lens mount and a collection of electronic elements required for operation of the sensor package.
Generally, a field of regard of an ambient light sensor package may be increased in three ways: if a lens of fixed focal length is present by widening or enlarging a pixel array; by decreasing a distance between a pixel array and a lens within the ambient light sensor package while also reducing the focal length of the lens accordingly; and where no lens is used by reducing the distance between the pixel array and the window aperture or increasing the window aperture size.
Referring to FIG. 1, an effect of altering the number of pixels in a pixel array on the field of regard of a sensor package is shown.
The sensor package 10 comprises a sensor array microchip 12 and a lens 14. The sensor array microchip comprises a substrate 16 and a series of pixels arranged in a pixel array 18. A normal 20 is shown that is perpendicular to the pixel array 18 and that passes through the center of the lens 14. In addition, a field of regard is defined by the outer boundaries 22a, 22b. The field of regard has a half-angle 24 and is determined by the relative arrangement of the pixel array 18 and the lens 14. The field of regard of the sensor package 10 indicates the maximum angle at which light incident on the lens 14 may still be sensed by the pixel array 18.
If the pixel array 18 is extended to include all the pixels of an extended pixel array 26 then the half-angle of the field of regard is increased to an extended half-angle 30. This can be seen by extended limits 28a, 28b of a field of regard defined by the relative arrangement of the extended pixel array 26 and the lens 14.
However, by increasing the number of pixels arranged on the substrate 16, the size of the microchip, and therefore the sensor package, may be increased. This is because the area of substrate surrounding the pixel arrays 18, 26 is used to hold support electronic components necessary to operate the sensor array microchip 12. If the size of the pixel array 18 is increased to include the pixels of the extended pixel array 26 then the inclusion of support electronics on the substrate 16 would result in a larger microchip.
Alternatively, if the size of the microchip is maintained then there is less area of substrate available on which to place any support electronics components.
Referring to FIG. 2, the effect of moving a lens closer to a pixel array on the field of regard of a sensor package is shown. A prior art sensor package 40 comprises a sensor array microchip 42 and a lens 44. As with the sensor package of FIG. 1, the sensor array microchip 42 comprises a substrate 46 and a pixel array 48. A normal 50 is shown perpendicular to the pixel array 48 and extending through the center of the lens 44.
The field of regard defined by the relative arrangement of the pixel array 48 and the lens 44 is shown by field of regard limits 52a, 52b defining a half-angle 54.
If the lens 44 is replaced with a lens 56 positioned closer to the pixel array 48 then the field of regard is widened. This is shown by the extended field of regard limits 58a, 58b that define an extended half-angle 60 increased relative to the half-angle 54.
However, widening the field of regard by moving the lens 44 closer to the pixel array 48 results in an ambient light sensor package that is difficult to manufacture. This is because bringing the lens 44 closer to the pixel array 48 requires greater accuracy of lateral placement and orientation of the lens 44. That is, if the center of the lens 44 is positioned slightly to one side of the normal then the effect would be to skew the field of regard of the sensor. This effect is more pronounced with a lens positioned closer to the pixel array 48 and may adversely affect the operation of an ambient light sensor package.
When moving the lens 44 closer to the pixel array 48 the radius of curvature of the lens 44 must be adjusted to change the optical power of the lens 44. This is required to alter the focal length of the lens 44 to maintain the level of focus of light onto the pixel array 48.
The skilled person will understand that FIGS. 1 and 2 are sections through ambient light sensor packages and as such only show the dimensions of pixel arrays in one direction. Pixel arrays may be arranged as squares, rectangles or other shapes on a substrate. The same principles for field of regard and half angle apply to a section through a sensor package taken at right angles to that of FIGS. 1 and 2. The skilled person will therefore appreciate that the shape of the field of regard is dependent on the shape of the pixel array.
Proximity sensor packages generally comprise at least one light radiating source and at least one radiation sensitive element or pixel. The at least one light radiating source may for example be an infra-red light emitting diode (LED) and the at least one light sensor may for example be a photodiode. The LED transmits infra-red light and the detection of reflected infra-red light onto the at least one pixel signals the proximity of an object to the proximity sensor package.